1. Field of the Invention
The present invention relates to an LED drive circuit which causes a light emitting diode (LED) to blink periodically to reduce power consumed by the LED.
2. Description of the Related Art
A conventional LED drive circuit such as shown in the circuit diagram of FIG. 15 is known. That is, a power supply of a voltage VDD [V] is connected to a power supply terminal 10, and a constant current generation circuit 15 operates in such a manner that a voltage difference between an output voltage Vref [V] of a reference voltage circuit 11 and a voltage Va [V] across a resistor 13 is amplified by an error amplifier 12 to control a gate voltage Verr for a transistor 14 so that Vref−Va=0.
In this drive circuit, LEDs 19 and 20 are respectively connected to two output terminals 1 and 2.
If the resistance value of the resistor 13 is R13 [Ω], a current I=Va/R13 [A] flows through the resistor R13. The same current as that flowing through the resistor R13 also flows through transistors 14 and 16. If all of transistors 16 to 18 are identical in characteristics, a current mirror circuit 21 causes the same current as that flowing through the transistor 16 to flow through each of the transistors 17 and 18, thereby lighting the LEDs 19 and 20.
That is, currents Iout1 and Iout2 flowing through the LEDs 19 and 20 are given by the following equation (1):Iout1=Iout2=Va/R13[A]  (1)
Therefore the currents caused to flow through the LEDs 19 and 20 can be set to a desired current value by adjusting the value of the resistor 13 or the output voltage value of the reference voltage circuit 11.
If power consumed by the reference voltage circuit 11 and the error amplifier circuit 12 is negligibly small in comparison with power consumed by the LEDS, power Pd consumed by the LED drive circuit shown in FIG. 15 is given by the following equation (2):Pd=VDD×Va/R13×3[A]  (2)
To reduce power consumption in the conventional LED drive circuit, however, it is necessary to reduce the LED current. If the LED current is reduced, a problem of reduction in luminance of the LED arises.